There is a youtube video on checking them. Basically remove the sensor and clamp it into a vise. There are either 1 or 3 wires. If 3 wires it is heated, put one meter lead on the odd colored lead, the other lead to the chassis of the sensor. Set the voltmeter to low voltage, if you go to ohms you will destroy the unit. Then heat the business end with a torch, the voltmeter should read just under 1 volt if it is good. In the video, the sensor is heat to red hot. The sensor makes a comparison between inside and outside O2. Make certain the outside vent is not clogged.